There are several neutrophil-associated pulmonary diseases, including chronic obstructive pulmonary disease (COPD), chronic bronchitis (CB), pulmonary emphysema, α-1 anti-trypsin deficiency, cystic fibrosis, idiopathic pulmonary fibrosis, and adult respiratory distress syndrome.
In chronic bronchitis there is cough and mucous hypersecretion with enlargement of tracheobronchial submucosal glands and a disproportionate increase of mucous acini. CD8+ve lymphocytes predominate over CD4+ve cells and there are increased numbers of subepithelial macrophages and intra-epithelial neutrophils. Exacerbations of bronchitis are associated with a tissue eosinophilia, apparent absence of IL-5 protein but gene expression for IL-4 and IL-5 is present. In small or peripheral airways disease, there is inflammation of bronchioli and mucous metaplasia and hyperplasia, with increased intraluminal mucus, increased wall muscle, fibrosis, and airway stenoses (also referred to as chronic obstructive bronchiolitis). Respiratory bronchiolitis involving increased numbers of pigmented macrophages is a critically important early lesion. Increasingly severe peribronchiolitis includes infiltration of T lymphocytes in which the CD8+ subset again predominates. These inflammatory changes may predispose to the development of centrilobular emphysema and reduced FEV1 via the destruction of alveolar attachments.
In emphysema there is abnormal, permanent enlargement of airspaces distal to the terminal bronchiolus (i.e. within the acinus) accompanied by destruction of alveolar walls and without obvious fibrosis. The severity of emphysema, rather than type, appears to be the most important determinant of chronic deterioration of airflow, and in this there may be significant loss of elastic recoil and microscopic emphysema prior to the observed macroscopic destruction of the acinus. Airway obstruction and chronic expectoration, as well as accelerated decline in lung function, are associated with increased numbers of neutrophils in the sputum of smokers and ex-smokers.
COPD is a complex condition with an imprecise definition, which makes a definitive morphological description difficult. Chronic bronchitis and COPD are caused by a predominantly neutrophilic bronchial inflammation with sputum production and periodic flares of increased cough and sputum volume, often associated with shortness of breath. COPD generally refers to those patients that have associated parenchymal emphysematous changes with loss of lung compliance, increased air trapping, and reduced expired lung volumes and expiratory flow rates. It is believed that emphysematous changes are also the result of neutrophil-induced parenchymal damage. Barnes, P. J. Chest 117:10S014S (2000). Neutrophils are present in large numbers in bronchoalveolar lavage and in sputum of patients with chronic bronchitis and COPD, and sputum neutrophils correlate inversely to the FEV1 in patients with COPD. Yamamoto, C. et al., Chest 112:505-510 (1997); Peleman, R. A. et al., Eur. Resp. Journal 13:839-843 (1999). Interleukin-8 (IL-8) is believed to play a primary role in this activation. Peleman, R. A. et al., Eur. Resp. Journal 13:839-843 (1999); Pesci, A. et al., Respiratory Medicine 92:863-870 (1998).
IL-8 is a chemoattractant and granule release stimulus for neutrophils. Increased concentrations of IL-8 are found in the sputum and bronchoalveolar lavage fluids of patients with COPD and chronic bronchitis, and these concentrations correlate with the number of neutrophils recovered. Bronchoalveolar lavage fluid IL-8 concentrations are higher in cigarette smokers than in nonsmokers, and cigarette smoke concentrate induces IL-8 release from cultured human bronchial epithelial cells. “Cigarette Smoke Induces Interleukin-8 Release from Human Bronchial Epithelial Cells,” Am. J Respir. Crit. Care Med., 155:1770 (1990). Nicotine prolongs neutrophil survival by suppressing apoptosis. Aoshiba, K. et al., J. Lab Clin. Med. 127:186-194 (1996).
The role of glucocorticoids in the treatment of COPD is controversial. As opposed to the eosinophilic inflammation of asthma, this type of inflammation is relatively glucocorticoid resistant when treated with either systemic or topical glucocorticoids. Studies have shown that inhaled steroids have little anti-inflammatory effect, and that the inflammatory process in COPD is resistant to the anti-inflammatory effect on glucocorticoids. Barnes P. J., Am J. Resp. & Critical Care Med. 160:S72-9 (1999); Keatings, V. M et al., Am J. Resp. & Critical Care Med. 155:542-548 (1997); Culpitt S. V., et al., Am J. Resp. & Critical Care Med. 160:1635-1639 (1999). Furthermore, studies have shown that glucocorticoids have no effect on the long-term decline of pulmonary function in patients with COPD. Vestbo, J. et al., Lancet 353:1819-1823 (1999). Sputum IL-8 concentrations correlate with long-term decline of pulmonary function in COPD, and glucocorticoid treatment in these patients does not result in reduction of neutrophils or IL-8, even in those patients with COPD who have eosinophils in their sputum. It has been recently shown that some patients with severe asthma have only neutrophils in their sputum, and these patients do not respond to glucocorticoids. At this time, there is no effective anti-inflammatory therapy for these patients.
Other than smoking cessation, there is no established treatment for the neutrophilic inflammation in chronic bronchitis and COPD, nor the progressive pulmonary function decline in patients with COPD. Because the majority of these patients have smoking as an etiologic factor in their disease, smoking cessation has been the only preferred treatment. Hurd, S. et al., Chest 117:1S-4S (2000). Unfortunately, smoking cessation is not a realistic therapeutic remedy in the vast number of patients effected. Despite many different types of intervention programs for nicotine dependence, the long-term abstinence rate for smokers is only about 20-25%. This means that over three fourths of these patients (about 25% of the US adult population) continue to smoke and develop neutrophilic airway inflammation, and risk sustaining variable degrees of progressive airway and parenchymal damage. Furthermore, smoking cessation may take years to accomplish or not be possible at all for most patients who smoke. Finally, there is evidence that in some patients, especially those with COPD, that there can be ongoing neutrophilic airway inflammation that continues after smoking cessation. Rutgers, S. R. et al., Thorax 55:12-18 (2000); Maziak, W. et al., Am. J. Resp. Crit. Care Med. 157:998-1002 (1998); Turato, G. et al., Am. J. Resp. Crit. Care Med. 152:1262-1267 (1995).
Pulmonary emphysema is defined as an abnormal, permanent enlargement of the air spaces distal to the terminal bronchioles with destruction of the air space walls, but without obvious fibrosis. Central to the pathogenesis of emphysema is lung destruction resulting from inadequate protection of the alveoli against enzymes released by inflammatory cells. The proteolytic enzymes implicated in the development of emphysema include neutrophil elastase, a major serine protease contained within neutrophil granules.
In α1-Antitrypsin Deficiency disease, there is a deficiency of the protein neutralizing neutrophil elastase, namely α1-antitrypsin. Moreover, there is evidence that the numbers of neutrophils migrating into the lung parenchyma is increased. Because of the inadequate defenses against neutrophil elastase, uninhibited neutrophil elastase is free to interact with substrates and inflammatory cells activating them. The result of this process is proteolytic destruction by neutrophil elastase of the fragile alveolar walls culminating in a gradual destruction of alveoli. This process is strikingly accelerated in cigarette smokers.
Cystic fibrosis is a common hereditary disorder of Caucasians and represents the most aggressive form of bronchitis known. Respiratory manifestations of cystic fibrosis develop at an early age, even in the first year of life. Frequent respiratory infections occur with production of thick, sticky sputum. The clinical course is punctuated by acute exacerbations of inflammation and infection of the airways with progressive deterioration of airway function. The pulmonary inflammation typical of cystic fibrosis resembles that of α1-antitrypsin deficiency and the epithelial surface of the lung in both disorders is burdened by neutrophil elastase and oxidants. However, in cystic fibrosis the bronchial inflammation is much more intense and the numbers of neutrophils in the airway epithelial lining fluid may be 500 times greater than in normal individuals. Although the pathogenesis of airway inflammation in patients with cystic fibrosis remains the subject of debate, nonetheless neutrophil elastase is implicated as playing a critical role. In cystic fibrosis, the normal protective defenses against proteases are intact, but the inflammation on the airway epithelial surface is so intense that these defenses against neutrophil elastase are overwhelmed and rendered ineffective.
The bronchitis associated with cystic fibrosis is the most aggressive form of bronchitis known, but the bronchitis associated with cigarette smoking is by far the most common. The population of inflammatory cells includes increased numbers of neutrophils, and they release an increased burden of oxidants and proteases including neutrophil elastase on the airway epithelial surface. Here, as in cystic fibrosis, it is assumed that the defenses of the airway against proteases are overwhelmed by the inflammation, albeit not to the extent as in cystic fibrosis.
The etiology of idiopathic pulmonary fibrosis is unknown, however, evidence exists that these patients show an inflamed lower respiratory tract even before the development of frank fibrosis. Among the inflammatory cells, neutrophils are prominent. It is believed that activated inflammatory cells damage alveolar structures by releasing oxidants and proteases, and that these reactants play a major role in injury to the epithelium and endothelium in idiopathic pulmonary fibrosis.
Considerable evidence links inflammation to the pathology of Acute (Adult) Respiratory Distress Syndrome (ARDS). Moreover, the neutrophil, in particular, with its extensive armamentarium of toxins, which can injure and destroy host tissue, has been implicated as an important mediator of ARDS.
Therefore, there is a driving need to develop new and effective treatments for neutrophil-associated pulmonary diseases such as COPD, CB, pulmonary emphysema, α-1 anti-trypsin deficiency, cystic fibrosis, idiopathic pulmonary fibrosis, and adult respiratory distress syndrome.